1. Field of the Invention
The present invention relates to a charge supply device of the contact type in use with an image forming apparatus, such as a printer, a video printer, a facsimile, a copying machine, or a display, and more particularly to a contact charging device and a contact transfer device in use with the image forming apparatus.
More specifically, the invention relates to a contact charging device for charging or discharging a member to be charged by bringing a charging member applied with an external voltage into contact with the member to be charged, and a contact transfer device for transferring developer onto a transferred-image recording media from the member to be charged when the transferred-image recording media passes through a space between a transfer member applied with an external voltage and the member to be charged. The charging member and the transfer member will be referred to as a "contact member" hereinafter.
2. Discussion of the Conventional Art
In the image forming apparatus based on the electrostatic electrophotography system, a latent electrostatic image is formed on a photoreceptor drum, toner is attracted to the latent image, and the toner image formed is transferred onto a transferred-image recording media.
The photoreceptor drum used in the electrophotography system is constructed such that an underlayer is formed on the surface of a drum as a base, and a photoreceptor layer whose electric conductivity varies in response to light is formed on the underlayer. In some cases, the photoreceptor layer is layered directly on the surface of the drum, not using the underlayer.
The drum is made of such a metal as to have a required rigidity and to allow a hard, electric insulating film to easily be formed on the surface thereof. Such a metal is typically aluminum. The underlayer is usually an oxide film or an electric insulating film, which is formed on the surface of the drum.
Organic or inorganic material, used for the photoreceptor layer, exhibits an electrical insulation to such a degree as to retain charges when it is not exposed to light, and as to release charges therefrom when it is exposed to light. When a material making the photoreceptor layer is an organic material, the photoreceptor layer is formed by immersing a drum with the underlayer formed thereon in a preparation liquid that is formed by dissolving the organic material into a solvent. When a material making the photoreceptor layer is an inorganic material, the photoreceptor layer is formed by vapor depositing the inorganic material on the underlayer formed on the drum.
The photoreceptor drum thus constructed is charged at a fixed potential by a corona charging device, a contact charging device, or the like. Under this condition, the photoreceptor layer on the drum is exposed to light beams or an optical image patterned according to image data in order to form a latent electrostatic image thereon. The electric resistance values in only the portions of the photoreceptor layer which are exposed to the light are selectively reduced, so that charges present on the surface disappear and the potential thereon drops.
Charged toner is brought into contact with the photoreceptor layer bearing the latent electrostatic image thereon, so that the toner is attracted to only the portions exposed or not exposed to the light by an electrostatic force, thereby forming a toner pattern on the photoreceptor layer.
Then, a transferred-image recording media is moved toward the surface of the photoreceptor drum in synchronism with the rotation of the drum bearing the toner image on the surface thereon. Then, the transferred-image recording media is charged in the polarity opposite to the polarity of the charged toner. The toner pattern on the drum is attracted to the transferred-image recording media, so that the toner pattern is recorded on the transferred-image recording media.
The device for charging the photoreceptor drum, the discharging device for removing the residual charge on the drum, and the transferring device for transferring the toner pattern on the transferred-image recording media belong to the devices for supplying and removing charges to and from the drum. By convention, the called corona charging device, which utilizes particles charged by the corona discharging, is used for those devices.
The use of the corona charging devices inevitably generate ozone, which contaminates air. To avoid this, contact charging devices and contact transfer devices, which generate an extremely small amount of ozone, have been used recently.
In the contact charging device, a brush of conductive fibers or a roller made of conductive elastic material, being applied with external voltage, is brought into contact with the surface of the photoreceptor drum, while the contact member, i.e., the brush or the roller, is being moved relative to the drum. A minute spark is generated in a gap between the contact member and the drum surface, which is formed when they approach to each other or separate from each other. Through this process, the photoreceptor drum is charged.
In the contact transfer device, a brush of conductive fibers or a roller made of conductive elastic material, being applied with external voltage, is made to approach to each other in a state that a transferred-image recording media is placed therebetween. At this time, the contact member is being moved relative to the drum. A minute spark is generated in a gap between the contact member and the transferred-image recording media, which is formed when they approach to each other or separate from each other. Through this process, the image on the photoreceptor drum is transferred onto the transferred-image recording media.
When the transferred-image recording media is not present between the contact member and the drum surface, a voltage for cleaning the contact member (causing the toner adhered to the contact member to move to the drum surface) is applied thereto to clean the contact member.
The discharge phenomenon is used also in the contact charging device and the contact transfer device. Accordingly, a voltage of approximately 0.5 to 1.5 kV, lower than that for the corona discharge, is applied between the contact member and the photoreceptor drum. To keep the breakdown voltage of 0.5 to 1.5 kV, the voltage must be properly distributed into the photoreceptor layer and the underlayer so as not to damage them.
Where the photoreceptor layer has a defective part or parts and a pinhole or holes with dusty material, i.e., foreign material attaching thereto, and those provide current paths, the current concentrically flows through those current paths.
When the contact member comes in contact with the defective parts or the pinholes, the voltage applied to the contact member causes current to flow to the conductive paths formed by the defective parts and the foreign materials in the pinholes or the defective parts since the impedance of the conductive paths is lower than that of the remaining portions of the photoreceptor layer. At this time, no discharge phenomenon occurs between them, or the contact member and the photoreceptor layer.
If the current flowing into the pinholes exceeds a current value predetermined for the related circuit, the voltage applied to the contact member, or the charging member, drops, so that no discharge takes place in the gap between the charging member and the photoreceptor layer. As a result, only the contact area of a part of the photoreceptor layer, which includes the pinholes and extends in the axial direction, and is in contact with the charging member, suffers from poor discharge. The poor discharge part appears as a white stripe in the normal development and as a black stripe in the reversal development. This considerably reduces the image quality.
Additionally, the current concentrically flowing into the extremely small areas is excessively large. This excessively large current heats the charging member in these areas and the foreign material in and around the pinholes. The material of the charging member is changed in quality and the pinholes of the photoreceptor layer is enlarged, possibly creating serious problems in the machine.
To solve the problems, the techniques to limit the lower limit value of the resistance of the charging member have widely been used as disclosed in Published Unexamined Japanese Patent Application Nos. Sho. 56-132356, Sho 58-49960 and Sho. 64-73365, for example. In one of the techniques, the volume resistivity of the charging member is set within 10.sup.5 to 10.sup.11 (.OMEGA.cm).
Techniques using the charging member, which is multilayered such that the volume resistivity of the outer layer thereof is larger than that of the inner layer, have been proposed in Published Unexamined Japanese Patent Application Nos. Sho. 64-73364 and Hei. 4-138477, and U.S. Pat. No. 5,126,913, for example.
Specifically, Published Unexamined Japanese Patent Application No. Hei. 4-138477 discloses a charging member having such a multilayered structure that the surface layer exhibits an anisotropic property of conductivity and has 10.sup.5 .OMEGA. or more along the surface thereof. U.S. Pat. No. 5,126,913 uses a power source of such a large capacity as to keep the power source output constant even if the current concentrates at the pinholes.
Many proposals have been made on the technique to lay an underlayer between the photoreceptor layer and the drum body. Those proposals discuss mainly improvements on the adhesion of the photoreceptor layer to the conductive layer or the drum, the coating of the photoreceptor layer, and the dark/light decay characteristics of the photoreceptor layer. Among those proposals, Published Unexamined Japanese Patent Application No. Sho. 61-179464 discloses a technical idea in which the lower limit value of the divided charged potential for the underlayer (or the intermediate layer) is set at 1 V, in order to suppress the formation of pinholes in the photoreceptor layer by the discharge.
Also in the contact transfer device, as in the contact charging device, when the current flowing into the pinholes exceeds a current value predetermined for the related circuit, the voltage applied to the transferred-image recording media drops, so that no discharge takes place in the gap between the transferred-image recording media and the transfer member. As a result, only the contact area of a part of the photoreceptor layer, which includes the pinholes and extends in the axial direction suffers from poor transfer. The transfer member is changed in quality and the pinholes of the photoreceptor layer are enlarged, possibly creating serious problems in the machine.
The inventors of the present Patent Application, after carefully studying the problems of those devices in connection with the conventional techniques, confirmed the following facts. The conventional technique cannot suppress or eliminate such a phenomenon that when the contact member comes in contact with the defective part and/or the pinholes of the photoreceptor layer, a current, which is in excess of a current value calculated on the basis of the volume resistivity of the contact member, flows into the defective part and/or the pinholes. Accordingly, poor charging or transfer inevitably takes place over the entire contact area across the photoreceptor layer with the contact member. The resultant image is poor. Further, a situation that the current flowing into the pinholes heats the contact member or the pinholes of the photoreceptor layer, thereby deteriorating the contact member or enlarging the pinholes, is also inevitable.